Instrumented rolling bearings which are intended to be mounted on a shaft and which require the steering column to be dismantled when a defective element of a rolling bearing of this kind has to be changed are known from the prior art.
Equipment for detecting the rate of rotation and direction of a steering column shaft is known from document EP-A-0,443,940. The steering column shaft is fitted with a rolling bearing comprising an elongate inner race and an outer race which is also elongate, with rolling bodies arranged between the two races. The inner race is secured to the shaft and the outer race is secured to the steering column casing. An encoder is mounted on the inner race. A sensor is secured to a removable sensor block and supported by the stationary outer race.
This type of equipment has certain drawbacks. The removable sensor block is clipped to the outer race of the rolling bearing of one of the steering column bearings, which means that this race has a shape specially adapted to receive the said block. The encoder ring is fixed to the rotating race of the same rolling bearing, which requires a race shape and dimensions adapted to receive the said encoder. The result of these two requirements is that use is made of non-standard bearings, which leads to an additional cost.
As only the sensor is removable, replacing a defective detection unit (sensor and encoder) therefore involves operations on the steering column mechanism in order to remove the rolling bearing fitted with the encoder.
As the sensor block is mounted on the steering column bearing, the variations of the loads applied to the said bearings may cause variations in the size of the radial air gap between the sensor and the encoder, and this detracts from the detection accuracy. In the event of a failure of the steering column bearing, and of this bearing being changed, the encoder cannot be recovered. The protection of the sensor and of the encoder against external contaminants is not entirely satisfactory. The detection equipment can be fitted only at the end of a steering column in so far as it is fixed to one of the rolling bearings that form the steering column bearing.
Other devices of the prior art propose detection systems in which the encoder is fixed to the rotating shaft of the steering column and in which the sensor is secured to the column casing.
In this case, the air gap between sensor and encoder is generally factory-preset at the time of fitting.
Apart from the problems already mentioned earlier, it is difficult to guarantee a precise air gap that gives a good signal when changing a defective sensor/encoder unit.